(1) Technical Field
The present invention relates to an exothermic conductive coating, particularly to an exothermic conductive coating for providing a conductive heating unit which can generate a uniform temperature distribution at any temperature up to about 450.degree. C. and the temperature of which is adjustable, and a conductive heating unit obtained therefrom.
(2) Background Information
Japanese Patent Publication No. 60-59131/1985 discloses a planar electric heating element comprising a synthetic resin film having a conductive fine powder of carbon, such as shell-like, flake-like, needle-like or fiber-like carbon, or graphite incorporated therein and electrode wires buried in the film at both ends in the longitudinal direction thereof. There is known a heating unit which comprises a solid lined with this element and the temperature of which can be increased to about 60.degree. C.
However, for the heating unit containing the carbon black or graphite powder and the synthetic resin, the distance between electrodes on a coating film is narrow, for example, and a large heating surface having a uniform temperature distribution can not be obtained. In the heating unit wherein the conductive fine powder such as the conventional carbon or graphite powder is used, there is utilized the tape-shaped heating element which is formed by melt extrusion from the synthetic resin having this powder incorporated therein. Heretofore a paste or coating containing this conductive fine powder has not been used nor such coating applied to a surface to prepare a heating unit having a large heating surface.
When heat radiation is is blocked from the heating surface, the conventional heating unit is in danger of local oxidation or damage by burning. Therefore, the temperature of this unit can only be increased to a temperature below about 60.degree. C.
In a conventional heating unit, a substrate 1 is lined with a planar heating element (tape) 2 as shown in FIG. 10(a). The supply of electricity through metal terminals 3 causes the heating part (element 2) to be heated to produce a temperature distribution 6 on the substrate as shown in FIG. 10(b).
Thus, for the heating unit containing the conventional conductive powder such as a shell-like, flake-like, needle-like or fiber-like carbon or graphite powder, a large heating surface having a uniform temperature distribution can not be obtained. When the substrate is coated with the paste or coating containing such an conductive powder, the thickness of the coating film must be precisely controlled. This requires that the coating be applied by a suitable machine to achieve the precise thickness, for example, of 1/10 to 1/100 mm. The coating can not be manually applied.
According to the conventional heating unit, the more electric current is supplied to the thicker portion when the thickness of the coating film is varied, and consequently the temperature of that portion is elevated. Further, the resistance slightly increases with an increase in temperature (FIG. 1(b)). Local overheating is therefore expected, when uneven action of heat radiation is exerted. In order to prevent this overheating, measures such as the use of thermostats and the incorporation of temperature controllers are taken. However, it is impossible to anticipate where heat radiation is locally prevented on the large surface. Moreover, it is impossible to estimate the number of such local portions and to mount a number of sensors thereto. Therefore, the planar electric resistance heating unit having these conventional conductive fine powders has not proven popular.
According to the prior art, the paste or coating to be precisely applied by a machine. In an electric resistance heating unit having a curved surface, an inner surface of a hole or an uneven surface on which the paste or coating can not be applied by machine, the local overheating as described above takes place. It is therefore very difficult to produce the electric resistance heating unit satisfactorily by the prior art.
Consequently, there has long been the need for an exothermic conductive coating or paste using improved conductive carbon material to provide an electric resistance heating unit with a large heating surface on which a uniform temperature distribution can be obtained, even if a substrate has a complex structure such as a curved surface, an inner surface of a hole or an uneven surface, and in which the substrate is coated with the paste or coating to a by hand or by impregnation, the local damage by melting or by burning does not take place, and the heating temperature can be freely controlled.